Evaluation of Ivermectin Efficacy and Anthelmintic Resistance in Naturally Occurring Gastrointestinal Nematode Infections of Sheep in Central Kashmir

Gastrointestinal nematode (GIN) infections in sheep cause substantial production losses, and increasing anthelmintic resistance threatens the sustainability of routine control programs. This study evaluated the field efficacy of injectable ivermectin against naturally occurring GIN infections in Kashmir Merino sheep reared under a semi-intensive system in central Kashmir. A total of 100 sheep meeting inclusion criteria (fecal egg count ≥150 eggs per gram [EPG] and no anthelmintic treatment for at least 8–12 weeks) were randomly assigned to an ivermectin-treated group (n=75) and an untreated control group (n=25). Ivermectin was administered subcutaneously at 200 μg/kg body weight. Fecal egg counts were determined on Day 0 and Day 14, and pooled fecal cultures were used for larval identification following WAAVP guidance. The mean fecal egg count reduction (FECR) based on individual animal data was 74.33% (95% CI: 70.55–78.11%), with a corresponding group-level FECR of 71.06%, indicating reduced ivermectin efficacy consistent with moderate anthelmintic resistance under field conditions. Larval cultures showed a shift in genus composition following treatment: before treatment, Haemonchus spp. predominated (55%) alongside Trichostrongylus spp. (26%), Chabertia spp. (13%), and others (6%); after treatment, Haemonchus spp. increased to 78% while Trichostrongylus spp. declined to 20% and other genera were markedly reduced. These findings highlight the need for strengthened resistance management in the region, including targeted selective treatment, rotation of anthelmintic classes, and routine monitoring of drug efficacy.